SOLVENT EXTRACTION EXPERIMENTS ON HAT CREEK COAL Submitted in partial fulfilment of the requirements of the course in Chemical Engineering leading to the degree of Master of Applied Science. Graduate Tear, Faculty of Applied Science, The University of British Columbia. September 1, 19J+6 Frank Ekman ACHBOWLEDGMENTS I am pleased to acknowledge the help and guidance of Dr. W. F. Seyer during the past year. I am also indebted to Mr. Sun Yip, II.A,Sc., for advice based upon preliminary work he and others did for Dr. Seyer. TABLE OF CONTENTS Page 1. Introduction 1 2. Theoretical Considerations 2 a. The Origin of Coal 2 (1) The Lignin Theory 2 (2) The Cellulose Theory 6 b. Bie Chemistry of Goal 7 3. Apparatus and Plan of Work 12 4. Experimental Results 14 a. Preliminary Analyses of the Coal 14 b. Atmospheric Extraction 15 c. Pressure Extraction 22 d. Other Solvents 30 e. Separation of Extract and Solvent 32 f. Time-Extraction Relationships 34 g. Ultimate and Calorific Analyses 35 5. Summary 39 6. Conclusion 40 7. Bibliography 41 LIST OF ILLUSTRATIONS Plate 1 - Electric Furnace Plate 2 - Pressure Bomb. li INTRODUCTION At Hat Creek, near Ashcroft, there is a large seam of lignite coal, This coal seam has a great economic asset; it can be mined by open-pit methods. There are, however, disadvantages. The coal* as mined, has too high an ash and water content and too low a calorific value to be commercially attractive. In addition, like all lignites, it slakes and crumbles on prolonged storage. Dr. Seyer proposed a research program with the object of removing these disadvantages and producing a product suitable to the market. A feasible plan appeared to be the solvent extraction of the coal. By this method, all or most of the organic substances could be separated from the inorganic material. This extracted organic matter could then be utilized as fuel or as road-paving material. It is not improbable that part, or all of the extracted material could be used as a source of chemicals, 5to«s resaarch has been devoted entirely to the solvent extraction of coal. Although the problem has not been explored very fully, owing partly to inadequate equipment, enough results have been obtained to Justify further investigation. 1 2 2. THEORETICAL C0HSIDERATI0M3 a) The Origin of Coal (1) The Lignin Theory Although papers on the various chemical aspects of coal date back at least 100 years; it was not until 1929 that a paper dealing with the chemistry of the origin of coal was publishes. In that year Hans Tropsch read a paper at the^University of Prague* expounding the theory that coal is derived from lignin alone, and from none of the other plant substances. He had amassed so many facts to support his contentions that his views, "The Lignin Theory," are generally considered the post plausible existing to-day. Tropsch divided coal into three sub-groups: 1) humus coal, 2) eapropel coal, and 3) mother of coal. Humus coal is that part of the coal derived from woody substances; sapropel coal is that part derived from fats and albumins; and mother of coal is mineral charcoal. Very little is known of mother of coal, or of mineral charcoal; Tropsch considered only the humus coals, which are, however, the most abundant. There are three characteristic stages in the formation of humus substance: a) In Peat - The humus substances here exist as humic acids, 1Problems in the chemistry of coal, Chemical Reviews, vol. 6, 1929. Feats are acidic and dissolve in cold alkali, b) In Brown Coals or Lignitic Coals The huraic acids of peats have now been transformed into humins, which are anhydrides and lactones of the humic acids. These are not soluble in cold alkali but can be dissolved by boiling in lye, a process which converts them to the sodium salts of humic acids. The addition of hydrochloric acid or of other mineral acids precipitates out the humic acids. This process can be reversed; humic acids heated to 250°G. form humins. c) In Hard Coals The humus portion of hard coals is not soluble in boiling lye. However, Fischer and Schraeder converted these humus substances to compounds resembling humic acids by auto-oxidation at higher temperatures. The conversion can also be effected by pressure oxidation.,in the presence of alkali, or by moderate oxidation with H2O2.. The ease of interconvertibility suggests that there is no fundamental alteration in chemical structure. When all these substances are further degraded, benzene carboxylic acids and lower aliphatic acids are obtained. One-third of the identifi• able acid mixture consists of benzol derivatives. These can be converted to aromatic carboxylic acids by heat and pressure. As these same condi• tions of heat and pressure are known to degrade aromatic compounds, it is concluded that coal is largely aromatic in. structure. Under pressure oxidation, lignin is the only one of the vegetable substances to behave in a like manner. Under the pressure-oxidation conditions already referred to, cellulose behaves quite differently to lignin. Kb humi.c acids are formed. 4 but large quantities of aliphatic acids such as acetic, oxalic, fumaric, and succinic are. In addition, there are formed unidentified non-volatile acids which yield furan upon further oxidation. Under similar conditions no furan can be obtained from coal or from lignin. As it has been demonstrated that the furan ring is stable under these conditions, Tropsch. declared this to be conclusive evidence that no furan exists in coal. Humic acids can be formed artificially from various substances other than phenols, for example, carbohydrates; but these reactions proceed most smoothly from the phenols. It is known that phenols can be formed directly from cellulose and carbohydrates, and it Is therefore assumed that the formation of phenols is a necessary intermediate stop. Tropsch did not think this significant; it is, however, the weak point of his theory and the point upon which his opponents have centered their attack. The work of biologists and biochemists has helped to establish Tropsch*s theory. They have shown: (1) Cellulose is consumed by bacteria during plant deca y and disappears as GGj, CH^, H2O, and water soluble acids. (2) Lignin forms huimic acids. (3) laxes and resins form bitumens. In 1917, Hose and Lisse examined fresh, half-decayed, and fully decayed wood. They showed that the methoxyl content doubles during the process of decay, a good indication of an increasing percentage of lignin, Cellulose, on the other hand, disappeared. Bray and Andrews showed that the lignin content of wood remained practically constant and that the loss of weight during decay was the same as the weight of cellulose 5 originally present. Biey also showed that methoxyl splits off from lignin in the process of decay, thus explaining the. absence of methoxyl groups in coal. These methoxyl groups are more resistant than cellulose, and, consequently, do not begin to split off until the cellulose is largely decomposed. Time alone cannot bring about the transformation of peat to hard coals; heat is also necessary. In Russia, there are brown coals which, from their geological strata, should have been converted to more mature coals. That they have not been converted is due to the fact that they had not been exposed to heat. Erdmann had demonstrated this fact in the laboratory by heating lignite with oner-half its weight of water in an autoclave for 100 hours at 280°G. The brown lignite containing 64 per cent carbon was converted to a black coal containing 91 per cent carbon. He concluded from this that a temperature of about 300°C. is necessary to convert low-rank coals to those of higher rank. Goal bitumen is generally assumed to be derived from the waxes and resins of the coal-forming plants. Tropsch has isolated C25» Qg], and acids from bitumen; higher acids are believed to exist. It is rather remarkable that odd-numbered acids from Cgj to are also found in beeswax. These odd-numbered acids are not found in animal and vegetable fats, in which the glycerides of lower even-numbered acids predominate. It has been shown by cracking paraffins that the odd-numbered compounds are much more stable than the even ones. This leads to the conclusion that all even-numbered acids originally present in the vegetable matter disappeared in the course of plant decay. 6 2) The-Cellulose Theory This theory is generally thought less reasonable than the lignin theory. Although there is sojfte evidence to support it, top- ranking coal chemists do not believe it to be sound, 2 E. Berl and W. Koerber attacked the lignin theory because Fischer and his co-wprkers had considered only cellulose coals. They produced cellulose coals by heating cotton linters with 0.05 H MaOH in o an autoclave at 32 5 0. From this coal they then obtained aromatic compounds by the usual pressure-oxidation methods. They believed these experiments provided the.lignin theory in error and that cellulose is •a the chief sources of at least some coals. However, Thiessenv has shown that, in the moderate conditions prevailing in Wisconsin, cellulose decays in the first 10 feet of a peat swamp. And as, furthermore, 30 feet of peat are required to produce one foot of hard coal, it would seem that even if the cellulose theory were accepted, cellulose coal would only exist on the top four inches of a coal bed. b) The Chemistry of Coal The ultimate composition of any coal can be determined by analysis, but these data do not give much clue to the chemical structures and functional groups present.